1. Field of the Invention
The present invention relates generally to methods for forming a vehicle rim, and more particularly to a method including the spin forming of a preform of a heat treatable alloy into a simple but controlled shape, followed by solution heat treatment using minimal constraining of the preform that allows for natural thermal deformation, followed by one or more spin forming operations in which the heat treated preform is shaped to the final configuration.
2. Description of Related Art
Aluminum alloy wheels have become a staple for automobile enthusiasts who wish to enhance the appearance of their vehicles. Today, many of the aluminum wheels are formed of three pieces, included two mating halves that form the support rim, plus a centerpiece that includes an aesthetically pleasing design. Three piece aluminum wheels are typically formed by cold working non-heat-treatable aluminum alloys such as 5052 and 5454 in the T0 temper condition. Because of the heat treatment distortion that accompanies the use of higher strength alloys, the trend has been to use cold work alloys that are easier to produce.
Three piece wheels that use cold work alloys have drawbacks such as strength deficiencies in the bolt flange, i.e., the rim half assembly to center. In addition, wheels made from cold worked alloys often suffer from variable strength properties from part to part and from location to location within a particular part. This lack of uniformity must be compensated for by thicker parts that cost more and add unwanted and unnecessary weight to the components. In the racing field, the additional weight can represent a slower overall vehicle. Also, thicker parts can be more difficult to shape requiring more extensive cold working, and thus more expensive to produce than thinner parts. Other limitations of cold worked three piece aluminum wheels include edge cracking which is common during forming and can propagate along the part if undetected.
As a result of these shortcomings, it has been suggested to use one piece aluminum wheels manufactured from higher strength heat treatable alloys such as 6061-T6 for racing applications and the like. The benefit of the higher strength alloys and weight savings overcomes the shortcomings addressed with respect to the cold work alloys referenced above. The heat treatable alloys have higher strength to weight ratios, and allow for deeper, wider parts because they do not work harden as rapidly in the zero condition. Heat treatable alloys are harder and therefore more resistant to denting and scratching of the finished part, which is very important to the consumers who purchase these high end wheels. These alloys can be shaped more easily in the softer, “0” condition and then hardened using one of several types of heat treatment to the harden the product. The strength of such a wheel is more uniform throughout the part, and is less dependent upon a part's width. Heat treated parts achieve full strength across the part's cross section, resulting in a safer product.
The difficulty with the use of heat treatable alloys is that the material experiences both hardening and deformation during the heat solution process, resulting in a deformed part that has been significantly hardened while requiring what may be substantial subsequent shaping as a result of the warping and distorting that the part experiences due to the exposure of very high temperatures used to treat the parts. Because the subsequent step of reshaping the hardened part was expensive and time consuming, this process has been deemed impractical for high production output of aluminum wheels.
It is also known to use a preform to create a one-piece wheel similar to a casting, where the center and rim are made of a single piece of metal. A cast log is severed to yield a billet which is then subjected to a series of hot forging operations to form the wheel center and the rim of the wheel. The hot forging operations are conducted at temperatures in the 850°–900° F. and require multiple steps to achieve the desired preform configuration. Thereafter, the forging is subjected to a trimming operation. It is noted that the forging operation can result in collapsing or deformation of the wheel, particularly where openings or thickness gradients exist in the wheel as might be found in particular esthetics designs or for functional purposes. The one piece wheel is subjected to a solution heat treatment after which final contouring and shaping is performed by additional forming steps. This methodology is described in the inventor's previous patent, U.S. Pat. No. 4,579,604, entitled “Method of Spin Forging a Finished Article” issued Apr. 1, 1986, the disclosure of which is fully incorporated herein by reference.
A byproduct of the forging operation just discussed is the need to constrain the wheel during the solution heat treatment to minimize the thermal deformation. The presence of significant post-thermal deformation requires that the hardened, forged one-piece wheel must be reshaped. However, the solution heat treatment process makes it very difficult and costly to perform constraints on the wheel during the heat treatment process, and thus the process has not heretofore been available to high production wheels.